ArXiv.org is a service of Cornell University that hosts an insane number of important research papers in electronic format in many scientific subjects (Physics, Math, Nonlinear Sciences, Computer Science, and Quantitative Biology are the primary areas). Here at UC Davis, we host a mirror of the Mathematics section of the arXiv, which is found at front.math.ucdavis.edu. This is the primary mirror of the Mathematics section of the arXiv, which means that most people in the world who are reading mathematics papers stored on arxiv.org are downloading them from this server.
It's pronounced "archive." The X is the Greek letter "Chi."